From what I understand, Audio Play in Background only works for an audio that started playing when the game was running. Then it'll stop and not play any other audio as soon as it is finished.
In my opinion, this is a feature that should be enabled for dev and user consent, instead of blocked, since run in background tend to be important. I know it has to be used with caution, but this caution could maybe be controlled from the framework side.
Anyway, that said, I use a plugin called "Run in Background". It's kinda tricky, but it sure works on Android Platforms (Browser) and Desktops, but doesn't on iOS.
https://www.construct.net/en/make-games/addons/462/run-background
It even have an exemple file, it's worth checking out.